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How to solve this diffraction question? Please help me.?
A plane wave of wavelength 590nm is incident on a slit with a width of a=0.4mm. A thin converging lens of focal length +70cm is placed between the slit and a viewing screen and focuses the light on the screen. (a) How far is the screen from the lens? (b) What is the distance on the screen from the center of the diffraction pattern to the first minimum?
1 Answer
- kirchweyLv 71 decade agoFavorite Answer
Odd, I always thought the light from the slits is focused, but I guess it's not since the rays are parallel.
A. Screen distance D = f = 70 cm.
B. m = 1
Θ = arcsin(m*lambda/(d)) = 1.47500053484167E-3 rad, 8.45113054259669E-2 deg
y = D*tan(Θ) = 1.03250112316824E-3 m
I don't like this question or my answer to it. There is some distance, between the slit and the lens, that is unaccounted for. That will move y further from the center. Also the deflected rays from the slit won't pass through the center of the lens so assumptions about equal angles when passing through the center are invalid. Possibly that means we are supposed to assume the lens is right next to the slit. If so, that should have been stated.
